1. Field of the Invention
The present invention relates to an exhaust-gas purification device for an internal combustion engine, and relates in particular to an exhaust-gas purification device that includes a three-way catalyst and a nitrogen oxide purification device for purifying of nitrogen oxide, and that has a function for evaluating the deterioration of the nitrogen oxide purification device.
2. Description of the Related Art
When the air-fuel ratio of the fuel mixture supplied to an internal combustion engine is set leaner than the theoretical air-fuel ratio (a so-called lean operation is executed), the volume of the nitrogen oxide (hereinafter called xe2x80x9cNOxxe2x80x9d) that is discharged with the exhaust gases tends to increase. Therefore, in accordance with a conventional, well known technique, to remove NOx from the exhaust, a purification device that incorporates a NOx absorbent is provided for the exhaust system of an internal combustion engine. The material that is employed as the NOx absorbent has the following characteristics: when the air-fuel ratio is set so that a leaner mixture is supplied than one that is theoretically correct, and the oxygen content of the exhaust gases is comparatively high (the exhaust contains a high percentage of NOx) (a condition hereinafter referred to as a lean exhaust-gas condition), NOx is absorbed, while when the air-fuel ratio is set so that a richer mixture is supplied than the theoretically correct one, and the oxygen content of the exhaust gases is comparatively low (a condition hereinafter referred to as a rich exhaust-gas condition), NOx is discharged. The NOx purification device that incorporates such an NOx absorbent is so designed that, during a rich exhaust-gas condition, the NOx released by the NOx absorbent is reduced by HC and CO and is discharged as nitrogen gas, while the HC and CO are oxidized and are discharged as steam and carbon dioxide.
The volume of the NOx that the NOx absorbent can remove is naturally limited, and this limited value tends to become smaller as the NOx absorbent deteriorates. Therefore, a well known conventional method for evaluating the condition of the NOx absorbent has been disclosed (JP-A-10-299460). In this publication, oxygen sensors are provided upstream and downstream of the NOx absorbent device, and the air-fuel ratio is increased in order to discharge the NOx that is held by the NOx absorbent. The deterioration of the NOx absorbent is then determined by measuring the time that elapses from the time at which the value of a reading obtained by the upstream oxygen sensor indicates a rich air-fuel ratio, until the value of a reading obtained by the downstream oxygen sensor is representative of the rich air-fuel ratio.
Lean internal combustion engine operations are not constantly employed, and in accordance with the operating condition, stoich operations, for which the theoretical air-fuel ratio is set, or rich operations, for which the air-fuel ratio that is set is leaner than the theoretical air-fuel ratio, are also performed. Therefore, normally, not only is a NOx purification device employed, but also a three-way catalyst having an oxide reduction function. When a three-way catalyst is employed, since it must be activated as early as possible after the engine is started, the catalyst is positioned so it is nearer the combustion chamber of the engine than the NOx purification device, which is located downstream of the three-way catalyst because the NOx absorbent has low heat resistance. Thus, the following problem is encountered with the conventional method.
When deterioration of a three-way catalyst has occurred, and when the air-fuel ratio is changed from a lean air-fuel ratio to a rich air-fuel ratio, the time at which, downstream of the three-way catalyst, the reduction of oxygen density is required to be performed is earlier, and the densities of the HC and the CO that perform the reduction function are increased. Therefore, the time required for the reduction changes, even through the volume of the NOx absorbed by an NOx absorbent is the same, and a precise evaluation of the deterioration of an NOx purification device can not be performed.
It is an object of the present invention to provide an exhaust gas purification device that can perform a precise evaluation of the deterioration of an NOx purification device, even when it is located downstream of a three way catalyst.
To achieve the above object, according to a first aspect of the invention, an exhaust-gas purification device, which is provided for the exhaust system of an internal combustion engine, comprises:
nitrogen oxide purifying means for absorbing nitrogen oxide in exhaust gases in a lean exhaust-gas condition;
a three-way catalyst, which is provided upstream of said nitrogen oxygen purifying means;
a first oxygen sensor, which is provided between said nitrogen oxide purifying means and said three-way catalyst, for detecting the oxygen density in said exhaust gases;
a second oxygen sensor, which is provided downstream of said nitrogen oxygen purifying means, for detecting said oxygen density in said exhaust gas;
first deterioration determination means, for evaluating the deterioration of said three-way catalyst; and
second deterioration determination means, for evaluating the deterioration of said nitrogen oxide purifying means, based on the deterioration of said three-way catalyst and a first determination time period, after a lean air-fuel ratio of the exhaust gas has been shifted to a rich air-fuel ratio by an decrease in the air-fuel ratio of a mixture to be supplied to the internal combustion engine, from a time at which the value output by first oxygen sensor changes to a value indicating a rich air-fuel ratio until the value output by said second oxygen sensor changes to a value indicating a rich air-fuel ratio.
With this arrangement, the deterioration of the nitrogen oxide purifying means is evaluated based on the deterioration of the three-way catalyst and the first determination time extending from the time at which, after a lean exhaust gas mixture has been shifted to a rich air-fuel gas condition by an increase in the air-fuel ratio of a mixture to be supplied to the internal combustion engine, the value output by the first oxygen oxide sensor changes to one indicative of a rich air-fuel ratio, until the value output by the second oxygen sensor also changes until it is indicative of a rich air-fuel ratio. Therefore, an evaluation of the deterioration of the nitrogen oxide purifying means can be obtained, regardless of the degree to which the three-way catalyst has been deteriorated.
According to a second aspect of the invention, the exhaust-gas purification device for an internal combustion engine of the first aspect further comprises a third oxygen sensor, which is provided upstream of the three-way catalyst, for detecting the oxygen density in the exhaust gases. The first deterioration means evaluates the degree of deterioration of the three-way catalyst, based on a second determination time, which extends, after the lean exhaust gas has been shifted to the rich air-fuel ratio, from a time at which a value output by the third oxygen sensor is changed to a value indicative of the rich air-fuel ratio until the value output by said first oxygen sensor is changed to a value indicative of the rich air-fuel ratio.
According to this arrangement, an evaluation of the deterioration of the three-way catalyst is obtained, based on the second determination time, which extends from the time at which the air-fuel ratio is increased and the value output by the third oxygen sensor is changed to a value indicative of a rich air-fuel ratio, until the output value of the first oxygen sensor is also changed to a value indicative of a rich air-fuel ratio. Therefore, not only the deterioration of the nitrogen oxide purifying means can be evaluated, but also the deterioration of the three-way catalyst can be evaluated, and at the same time.
It is preferable that the second deterioration determination means correct the first determination time that it is extended as a result of the three-way catalyst being greatly deteriorated, and that when the corrected first determination time is shorter than a reference time, the second deterioration determination means determine that the nitrogen oxide purifying means has been deteriorated. Alternatively, the second deterioration means may correct the reference time so that it is reduced as a consequence of the three-way catalyst being greatly deteriorated, and, when the first determination time is shorter than the corrected reference time, the second deterioration determination means may determine that deterioration of the nitrogen purifying means has occurred. The reference time is set so it equals a time that corresponds to the time required when the purifying capacity of a nitrogen oxide purifying means is reduced until it is approximately 50% that of a brand new product.